The invention relates to portable power-operated tools in general, especially to power-operated portable chain saws. More particularly, the invention relates to improvements in portable power tools of the type wherein a sensor (such as a pivotable hand guard or the like) is provided on the housing to be displaced by hand in the event of an emergency (e.g., when the hand or hands of the operator cannot adequately control the movements of the housing) and to thereby actuate a brake for that part or for those parts which receive torque from the prime mover and can be disengaged from the prime mover by a clutch.
A portable power-operated chain saw of the above outlined character is disclosed in German Auslegeschrift No. 22 17 707. This chain saw employs a flywheel type clutch and a discrete hand brake for those rotary parts of the portable tool which are disconnected from the prime mover in response to disengagement of the clutch. In order to be effective, the brake must be designed in such a way that it can also brake the prime mover, at least until the clutch is disengaged. This necessitates the utilization of a large, complex, bulky and expensive brake and entails at least some delay in stoppage of the chain because the brake must decelerate large masses including those which are positively driven when the clutch is engaged as well as the prime mover and the parts which transmit torque from the prime mover to the clutch. It has been found that the just described portable chain saw cannot ensure a prevention of accidents in the event of excessive and uncontrollable stray movements of the housing, i.e., when it becomes necessary to displace the guard so as to disengage the clutch, even though the chain saw is equipped with costly, complex, bulky and expensive safety devices.
It was further proposed to provide a portable chain saw with an electric motor whose output member is directly connected with the part or parts which drive the chain. A large brake is provided to decelerate the motor in the event of an emergency. Such portable tools are relatively simple but the interval which elapses for deceleration of the motor and of the saw is too long.